J=lambda:map(int,input().split())
N,K=J()
A=list(J())
a=c=0
for d in range(29,-1,-1):
	a*=2;h={}
	for x in A:
		if x>>d in h:h[x>>d]+=1
		else:h[x>>d]=1
	s=c+sum((h[x]*h[x^a]if x^a in h else 0)if a else(h[x]-1)*h[x]for x in h)//2
	if s<K:a|=1;c=s
print(a)